/**
 * Definition for a binary tree node.
 * class TreeNode {
 *     val: number
 *     left: TreeNode | null
 *     right: TreeNode | null
 *     constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.left = (left===undefined ? null : left)
 *         this.right = (right===undefined ? null : right)
 *     }
 * }
 */
function convertBST(root: TreeNode | null): TreeNode | null {
  let sum = 0;
  return recur(root);
  function recur(root: TreeNode | null): TreeNode | null {
    if (root !== null) {
      recur(root.right);
      root.val += sum;
      sum = root.val;
      recur(root.left);
      return root;
    }
    return null;
  }
};